1. Field of the Invention
The present invention relates to a current driver for supplying a drive current to a display panel, such as an organic EL (Electro Luminescence) display device, or the like.
2. Description of the Prior Art
An organic EL element is an element which itself emits light according to the magnitude of en electric current input to the element. An organic EL display device including organic EL elements over a panel requires no backlight, and accordingly, the thickness thereof can be reduced. Further, the organic EL display device has no limitation on the viewing angle. Thus, the organic EL display device has been an expected next-generation display device which can replace liquid crystal display devices. Among various organic EL display devices, an active organic EL display device, including TFTs (thin film transistors) and organic EL elements which are provided to pixels arranged in a matrix, for example, over a panel on a one-to-one basis, has a response speed superior to that of a passive display device and therefore displays images with high quality.
The organic EL display devices have a driver circuit (current driver) for supplying a drive current to organic EL elements through signal lines and TFTs.
FIG. 12 is a circuit diagram showing part of a conventional organic EL display device disclosed in Japanese Unexamined Patent Publication No. 2000-276108. Among the components of the display device, a display panel 101 and a driver circuit 102 connected to the display panel 101 are shown in FIG. 12.
Pixel circuits provided over the display panel 101 each include a TFT 115 which is connected to a signal line 113 and opens/closes according to selection signal SCAN from a scan line 114, an organic EL element 119 connected to a source of the TFT 115, and a capacitor 117 for storage. One end of the capacitor 117 is connected to the source of the TFT 115, and the supply voltage of the display panel 101 is applied to the other end of the capacitor 117.
The driver circuit 102 includes a data register 108 for taking in image data D0 to D3, a shift register 109 for outputting shift clocks SF1, SF2, . . . each of which indicates the timing of taking image data into the data register 108, a latch circuit 110 for latching the image data taken in the data register 108, and a current mode D/A converter 126 for outputting to the signal line 113 an electric current whose magnitude is determined according to image data D0 to D3. The current mode D/A converter 126 is supplied with supply voltage Vdd. In the example described herein, image data reproduced by one pixel is 4-bit data.
FIG. 13A is a circuit diagram showing a structure of a conventional current mode D/A converter. FIG. 13B illustrates the relationship between image data input to the conventional current mode D/A converter and the electric current output from the D/A converter. In the example of FIG. 13, image data is 6-bit data (D0 to D5), although in the example of FIG. 12 the image data reproduced by one pixel is 4-bit data.
Referring to FIG. 13A, the conventional current mode D/A converter includes an n-channel MISFET 131, a bias line 137 which is connected to the gate electrode and drain of the MISFET 131, current sources S0, S1, . . . and S5 which are formed by n-channel current source MISFETs, and switches SWg0, SWg1, . . . and SWg5 which turn on/off according to image data D0 to D5 to allow/stop the flows of the output currents of the current sources S0, S1, . . . and S5. The drain and gate electrode of the MISFET 131 are connected to each other. During the operation of the D/A converter, a reference current flows through the n-channel MISFET 131. The gate electrodes of the current source MISFETs are commonly connected to the bias line 137. A resistor 135 is provided at an output terminal as necessary.
The current source Sx includes 2x current source MISFETs. That is, the current source S0 includes 1 current source MISFET, the current source S1 includes 2 current source MISFETs, . . . and the current source S5 includes 25 current source MISFETs. The current source MISFETs have the same size and same electric characteristics. The current source MISFETs and the MISFET 131 constitute a current mirror. When the switches SWg0, SWg1, . . . and SWg5 are ON, electric currents of I, 2I, . . . and 25I are output from the current sources S0, S1, . . . and S5, respectively, where I denotes a unit current. The output currents from the current sources connected to the switches which have been turned on according to image signals are summed and then output from the output terminal to the pixels. In this specification, the “reference current” means an electric current which serves as a source of a current mirror included in a D/A converter. The “unit current” means an output current of the current source MISFET at the least significant bit.
The electric currents flowing through the current source MISFETs are precisely equal due to the current mirror. Thus, as shown in FIG. 13B, in the conventional current mode D/A converter, the input data (grayscale value of image data) and the output current have the relationship of direct proportion.
In the above-described example, the image data is 6-bit data. In the case of n-bit image data (n is a natural number), there are n current sources S0 to Sn−1, and the current source Sn−1 includes 2n−1 current source MISFETs.
In the case of a current mode D/A converter provided in a driver LSI chip, the drain of the MISFET 131 is connected to an external resistor 133 which is provided outside the LSI chip. Alternatively, each of the current sources S0 to S5 may be formed by a single current source MISFET. In this case, the channel widths of the current source MISFETs which constitute the current sources S1, S2, S3, S4 and S5 are 2 W, 4 W, 8 W, 16 W and 32 W, respectively, where W is the channel width of the current source MISFET of the current source S0. However, when transistors have different sizes, a variation in the electric characteristics among the transistors becomes large. Therefore, the accuracy of output currents is higher in the example of FIG. 13.
In a conventional organic EL display device having the above-described structure, display is performed according to image data as described above.